The present invention is directed to a method of enhancing the thermal expandability of polystyrene, styrene copolymers, polymethyl methacrylate, methyl methacrylate copolymers, crosslinked variations thereof and interpenetrating networks thereof with carbon dioxide.
The preparation of molded foamed plastic articles is well known wherein plastic particles are impregnated with organic blowing agents, expanded to preliminary low density particles, placed into a mold and finally expanded to the desired fully expanded article. The most used organic blowing agent is n-pentane. Unfortunately, n-pentane is now considered to be hazardous in the quantities which are removed during the molding. Further, the residual pentane in the molded parts continues to escape into the atmosphere for an extended time after removal of the part from the mold.
The chlorofluoromethanes have been used to eliminate the problems with the pentanes. The recent findings that the usual chlorofluoromethanes cause loss of the ozone layers in the atmosphere, has made it necessary to look for replacements, such as the safer chlorodifluoromethane.
To help eliminate this problem, various inorganic blowing agents such as carbon dioxide, nitrogen, air and other pneumatogens have been tried.
U.S. Pat. No. 2,531,665 teaches to impregnate polymers at 100.degree.-25.degree. C. with inert gas under pressure followed by cooling and reheating in a mold to bring about expansion.
U.S. Pat. No. 2,848,427 teaches to impregnate polystyrene crosslinked with 0.01-0.25% DVB with CO.sub.2 under pressure at a temperature below the Tg until at least 2% CO.sub.2 was absorbed.
U.S. Pat. No. 3,243,485 teaches to impregnate films or sheets of polystyrene in dry ice.
None of these patents teaches to enhance the expandability of polymeric foam articles by the use of CO.sub.2 or any other pneumatogen.